a) Field of the Invention
The present invention relates to a brushless motor speed detector requiring no frequency generator (hereinafter referred to as FG) pattern or FG magnetization.
b) Description of the Related Art
An FG is widely used as a speed detector for various motors. FIGS. 10 to 13 show examples of the FG. In FIG. 10, a stator core 2 radially having a proper number of salient poles is secured onto a motor substrate 1 and a driving coil 3 is wound on each salient pole of the stator core 2. Moreover, an FG pattern 4 is formed on the substrate 1 like a rectangular wave along a circle concentric with and outside of the stator core 2. Furthermore, a motor driving IC 5 is arranged on the substrate 1.
A rotor 6 shown in FIGS. 11 and 12 is arranged on the stator core 2 and the FG pattern 4 so as to cover them. The rotor 6 comprises a compressed-cup-shaped rotor case 7 and an annular driving magnet 8 secured to the inside annular wall of the case 7. The rotor 6 is rotatably supported by the substrate 1 through a proper bearing at a proper interval between the external surface of each salient pole of the stator core 2 and the internal surface of the driving magnet 8. In FIG. 11, the bottom of the driving magnet 8 is magnetized at certain intervals in the circumferential direction to form an FG magnetizing section 9. The FG magnetizing section 9 faces the FG pattern at a certain interval.
In FIGS. 10 and 13, an output terminal 4a of the FG pattern 4 is connected to an input terminal of an FG amplifier 5a in the motor driver IC 5. Power is generated by each power generating section when the rotor 6 rotates and the magnetic flux emitted from the FG magnetizing section 9 crosses each power generating section of the FG pattern 4 and a signal with a frequency proportional to a motor speed is provided as an output from the output terminal 4a of the FG pattern 4. The output signal is waveform-shaped and amplified by the FG amplifier to serve as a speed detection signal. The speed detection signal is supplied as an output to a motor speed control circuit.
Therefore, the FG has widely been used as a general motor speed detector. To constitute the FG, an FG pattern and FG magnetization are necessary, thereby motor structure is restricted and cost also increases. A motor speed detector requiring no FG pattern or FG magnetization has been proposed. Some motor speed detectors are disclosed in the official gazettes of Japanese Patent Laid-Open No. 97294/1990, Japanese Utility Model Laid-Open No. 54499/1992, Japanese Patent Laid-Open No. 277191/1991, and Japanese Patent Laid-Open No. 193084/1992 in which the midpoint voltage of a driving coil and the counterelectromotive voltage of each phase driving coil are compared by a comparator to detect the alternate change point of the counterelectromotive voltage and provide a signal as an output corresponding to the detected point as a speed detection signal.
FIG. 14 shows an example of basic structures of the speed detectors.
In the case of the brushless motor, three-phase sine-wave signals corresponding to the relative positional relation between a stator having driving coils L.sub.U, L.sub.V, and L.sub.W and a not-illustrated rotor having magnetic poles are provided as as outputs from a not-illustrated Hall element to a brushless motor driving circuit 10 and the rotor rotates when a driving current is provided as an output from the brushless motor driving circuit 10 to the driving coils L.sub.U, L.sub.V, and L.sub.W.
The brushless motor is provided with a speed detector comprising a comparator group 16 and a logic circuit 17. That is, comparators 16a, 16b, and 16c for comparing the output voltage of each phase with the coil midpoint voltage are connected to the output terminals of the driving coils L.sub.U, L.sub.V, and L.sub.W respectively; AND gates 17a, 17b, and 17c using two of outputs from the comparators 16a, 16b, and 16c as their inputs are connected to the output terminals of the comparators 16a, 16b, and 16c respectively; and an OR gate 17d is connected to the output terminals of the AND gates 17a, 17b, and 17c.
Therefore, the output voltage of each phase and the coil midpoint voltage are compared by the comparators 16a, 16b, and 16c and the rectangular-wave signals shown by "a", "b", and "c" in FIG. 15 are provided as outputs from the comparators. The logical product of these signals is computed by the AND gates 17a, 17b, and 17c and the signals shown by "d", "e", and "f" in FIG. 15 are provided as outputs from the AND gates 17a, 17b, and 17c. Moreover, the logical sum of these signals is computed by the OR gate 17d and the speed detection signal shown by "g" in FIG. 15 is provided as an output from the OR gate 17d.
Then, motor speed can be kept constant by supplying the speed detection as an input signal "g" thus detected to a motor speed control circuit (not illustrated) so as to operate a control input of the brushless motor driving circuit 10.
Therefore, because the above speed detectors can detect speed without using a frequency generator (FG), it is possible to decrease the number of parts and decrease the cost.
The alphabet representing each waveform in FIG. 15 corresponds to that representing each point shown in FIG. 14.
The speed detectors disclosed in the above official gazettes have the disadvantages that it is necessary to use detection circuits including a comparator by the number of motor phases, and thereby a detection-signal processing circuit is complex and the accuracy of a detection signal is deteriorated due to the fluctuation of detection circuits of various phases.
That is, in the case of the examples shown in FIGS. 14 and 15, because three comparators 16a, 16b, and 16c are used, problems occur that the accuracy of a detection signal is deteriorated due to the characteristic difference between the comparators and the reliability is lowered. Moreover, because the comparator group 16 and the logic circuit 17 are used, a problem occurs in that the circuit is complex.
Even by using a detection circuit including a comparator only for one phase, it is possible to obtain a detection signal proportional to speed. In this case, however, the number of detection signals for one turn of a motor decreases greatly and the resolution lowers greatly.